I picked up an old Onkyo HT-R380 amplifier that was stuck in standby / protect mode so pulled it apart to find out what the problem was.
This is a typical fault condition with a lot of old amplifiers as something fails and triggers one of many protection circuits that switch the unit off.
If you’re fixing one of these yourself it won’t work with the back panel removed as it forms a ground connection between the standby board and the main board.
I’ve produced a YouTube video that takes you through how this Onkyo powers up and what causes it to switch into protect or standby mode.
I tracked the fault down to a Sanyo STK module that’s obsolete. I brought a cloned one from China and it was more faulty then the original faulty one!
Note: If you don’t know what you’re doing don’t start poking around inside these things. There’s +/- 50v on the circuit boards and live mains on things like the transformer. You could do a lot of damage to yourself or the amplifier.